Nostalgia Ninja
Something from the past you never knew knew existed, but realize as an adult you know everything about them.
Air Supply? Never heard of them. Oh, yeah, I recognize that song. Hey, I know all the words to this one. Wait, I know that one too. Holy crap, they also sang this one?! How do I know all the words of 11 of their songs and never knew who the hell they were?!?! That snuck up on me like a nostalgia ninja!